1. 3.1 – Atomic Theory and the History of the Atom
Physical Science 513 Unit 3
3.1 – Atomic Theory and the History of the Atom

2. Lesson Objectives Explain how atoms are related to elements
Describe the three sub-atomic particles
Describe the current model of the atom using correct terms such as “strong nuclear force” and “electron cloud”
Identify the historical events which led to our current understanding of the atom
Identify the six parts of Dalton’s Atomic Theory

3. The Atom Nucleus – in the center of atom
What is an atom?
The smallest particle of an element that retains the chemical identity of that element
Composed of subatomic particles: proton, neutron, and electron
Nucleus – in the center of atom
Protons – positively charged, have mass
Neutrons – neutral (no charge), have mass
Electrons – found outside of the nucleus
Negatively charged, almost no mass

4. Current Model of the Atom
● The nucleus (center) is composed of positively charged protons and
uncharged neutrons.
● A strong nuclear force keeps the positive charges in the nucleus from repelling each other.

5. Current Model of the Atom
● Electrons are moving in the space around the nucleus. We can’t predict where an electron will go or how it will get there.
● The area where there is the best chance of finding a particular electron is called an electron cloud.

6. Current Model of the Atom
There are up to 7 electron clouds around the nucleus.
Each one has a different energy level.
The farther away an electron is from the nucleus, the higher its energy level.
Electrons can change energy levels when heat is added or removed.

7. Current Model of the Atom
The 7 energy levels are divided into sub-levels called ORBITALS.
The orbitals are named s, p, d, and f.
They are different shapes.

8. Current Model of the Atom
● The first electron cloud only has 1 sub-orbital (s). It tends to be in a circular region around the nucleus. It holds 2 electrons.
● The second cloud has sub-orbitals s and p. Together they hold up to 8 electrons.

9. Current Model of the Atom
● The third electron cloud has sub-orbitals s, p, and d. This cloud can hold up to 18 electrons.
● The fourth energy level holds up to 32 electrons (s, p, d, f).

10. The History of the Atom 450 BC Democritus 1808 John Dalton
Proposed that all matter is composed of tiny particles called atomos (“indivisible”)
1808 John Dalton
Gathered experimental evidence for the idea that matter is composed of atoms

11. Dalton’s Atomic Theory (worksheet question #8)
Each element is composed of small particles called atoms.
All atoms of the same element are identical.
Atoms of different elements have different properties.
Atoms are rearranged in chemical reactions, but not changed by them.
Compounds are combinations of the atoms of two or more elements.
A compound is defined by the number, type and proportion of its atoms.

12. The History of the Atom 1896 J. J. Thomson Cathode Ray Tube Experiment
Stream of particles from a cathode flowed away from a negatively-charged plate
Suggested particles must have a negative charge
Thomson named these particles “electrons”

13. The History of the Atom 1909 Ernest Rutherford
Shot a beam of alpha particles (lightweight atoms) into a piece of thin gold foil
Most passed straight through; a fraction bounced back
Concluded that an atom is mostly empty space with a tiny solid core (nucleus)

14. THIS MODEL IS INCORRECT!
The History of the Atom
1911 Niels Bohr
Proposed that electrons move in orbits around the nucleus, just as planets revolve around the sun
Became known as the Planetary Model of the Atom
THIS MODEL IS INCORRECT!
However, Bohr did identify distinct energy levels around the nucleus

15. History of the Atom 1926 Erwin Schrödinger
Described electrons as moving in waves
Suggested that we can never tell exactly where an electron IS, only how much energy it has
Led to CURRENT model - the Quantum Mechanical Model of the Atom